The asthma drug Xolair is associated with a higher risk of heart attack, mini-stroke, chest pain and blood clots in the lungs and veins, among other problems, though the extent of increased risk is unclear, the U.S. Food and Drug Administration said on Friday.

The FDA said it has added information about the increased risk to the drug's label after analyzing findings from a five-year safety study submitted by the drug's manufacturer, Genentech, a unit of Roche Holding AG, and 25 clinical trials comparing Xolair to a placebo.

Xolair is an injectible drug that was originally approved in 2003 to treat moderate to severe asthma in adults and children aged 12 or over whose condition was not controlled by inhaled steroids.

The drug is also approved to treat chronic idiopathic urticaria, a form of chronic hives, in adults and adolescents. It is marketed jointly by Roche and Novartis AG. In 2013 it generated total sales of nearly $1.5 billion.

The companies said in a statement that they are committed to patient safety.

"We worked closely with the U.S. Food and Drug Administration on recent updates to the label and believe the revisions will provide healthcare professionals and their patients with the most up-to-date information to make informed healthcare decisions," they said in a statement.

The FDA said that although the data is suggestive of a serious safety signal, weaknesses in how the safety study was designed and carried out mean it is "unable to definitively confirm or determine the exact increased level of these risks with Xolair."

The problems involve the blood vessels supplying the heart and brain, the FDA said, and include a greater risk of pulmonary hypertension, or high blood pressure in the arteries of the lungs.

The agency said its review of the five-year study found no difference in the rates of cancer between those patients treated with Xolair and those not treated with the drug. However, due to the limitations of the study, the FDA said it could not rule out a potential cancer risk and has therefore added that information to the label as well.

Xolair is used in asthma patients who have elevated levels of a substance called IgE in their blood.

The FDA's announcement follows a 2009 statement in which it said it was evaluating interim safety findings from the five-year study that suggested an increased number of cardiovascular and cerebrovascular problems in patients taking Xolair.

The FDA at the time did not recommend any changes to the prescribing information.